1. Field of the Invention
The present invention pertains to a submersible mooring buoy adapted for transferring fluids to or from tank ships at anchorages which are ice infested or are exposed to severe weather.
2. Background
The development of petroleum resources in arctic regions and other locations which require the transportation of petroleum liquids and liquefied natural gas by marine bulk carrier vessels or "tankers" often poses the difficulty of providing a suitable anchorage for these vessels during the loading and unloading procedure. In this regard, several types of so-called single point moorings have been developed which are generally characterized by a floating buoy structure which is anchored in waters of suitable depth to serve as a mooring for a tanker during transfer of fluids between the tanker and onshore or offshore facilities. Such buoys are usually fitted with suitable conduits, such as hoses or articulated pipes, which may be connected to the tanker to transfer the fluids.
Several types of buoys have been developed with design features being dependent on site conditions including water depth, normal operating sea states and survival wave heights. The type of fluid being transferred and the fluid transfer capacity also dictate mooring buoy features. Known types of mooring buoys include the so-called catenary anchor leg mooring (CALM) which comprises a buoy type floating structure which is anchored to the seabed by plural radially extending catenary chain legs. The tanker or similar vessel is moored to the buoy by one or more elastic mooring lines and floating or buoy mounted hoses extend between the buoy and a fluid transfer manifold on the ship. Single anchor leg moorings have also been developed wherein a buoy structure is connected to the seabed by a single anchor leg which may be a flexible chain or a tubular type riser structure. Suitable universal joints or swivel devices permit lateral, pitch and rotational excursion of the buoy with respect to a seabed anchor structure.
However, known types of mooring buoys for transferring fluids to or from tankers are not suitable for placement in anchorages located in seas which have severe ice conditions or extreme wave heights. In this regard, it has been deemed desirable to develop a mooring buoy which may be submerged when not in use to minimize stress and strain on the buoy from severe sea states and to minimize damage or prevent total loss of the buoy in anchorages which are subject to severe ice coverage and/or ice movement at various times.
Locations wherein it has been deemed desirable to provide a mooring buoy which may be submerged to a suitable depth to avoid ice coverage or movement during inactive periods include proposed anchorages at certain points in ice infested waters, including the Bering Sea and the Chukchi Sea, for example. Certain proposals are being considered for the production of natural gas reservoirs on the North Slope region of the State of Alaska. These proposals contemplate the transfer of natural gas in liquid form onto tankers at one or more selected anchorages in the Bering Sea or the Chukchi Sea. Close to shore anchorages in these waters would be rather shallow and subject to severe sea ice coverage and movement. It is in this regard that the present invention has been developed with a view to providing a mooring buoy which may be used to transfer fluids to or from a floating tanker and which then may be submerged during periods of non-use to avoid damage due to ice formation and movement and other severe sea conditions.